The variable speed, condensing steam turbine has been evolved by the pressing need to substantially improve the overall performance and cost/effectiveness of the closed Rankine cycle power system.
The basic attractiveness of the Rankine cycle is widely known and includes multi-fuel operation, near silent running, fewer rotating parts/accessories, and reduced maintenance requirements. Considerable development work has been done on improved performance steam/vapor engines and systems during the past years by many companies without any notable successes or promising, competitive power systems against the conventional I.C. gas engine which has now long outlived its practical usefulness.
When carefully analyzed, the main problems blocking the acceptance of either the open or closed Rankine cycle system can be clearly defined and considered. By considering each component within the engine system, each can be evaluated and various options reviewed towards optimization of each component which can thereby lead to upgrading the full engine system.
As the basic engine/expander is considered it will be noted that these can be of three types, piston, rotary or turbine with each type having its own specific characteristics.
While most of the previous engine systems have been based on the multi-piston engine type, this must be considered as the least desirable steam/vapor expander due to high friction levels, excessive weight and complexity. The piston expander does offer the advantage of a ready made unit that can be readily reworked from an I.C. engine, but this is not a valid advantage when the previous deficiencies are all taken into account.
The rotary vane or roller type of expander is simple and effective but is also handicapped by moderate friction levels and full zone lubrication requirements. This type of expander is also limited in the range of speed/torque available, unless many multi-step units are utilized with multiple steam inlets provided to each unit with a central selector valve control.
It becomes fairly obvious that a nearly ideal steam/vapor expander would be a variable-speed turbine, since the turbine already offers the lowest possible-friction level and highest possible power-to-weight ratio among the three engine/expander types. If a simple variable speed/torque feature is built into a conical Tesla type of turbine an ideal steam/vapor expander will result which can be built at low costs for a wide variety of power applications. When most of the parts for such a turbine are fabricated from simple sheet metal stampings, cost can be held to a minimum for this power component.
The next major Rankine cycle component to be considered is the steam or vapor boiler or heating coil unit. The most effective type of steam generator is the horizontal multiple uniform tubing coil arrangement with central burner unit(s). The best design consists of using many spirally wound, small diameter tubing loops because the incoming fluid or water is quickly flashed over to steam or vapor within the many small diameter tubing loops.
Any type of large tank type boiler is definitely not acceptable for closed Rankine cycle systems because of the long heating times involved to bring the fluid to the boiling point, and due to upgraded safety and maintenance requirements for these newer systems.
The burner(s) arrangement within the steam/vapor generator is a most important factor in steam generation and requires careful analysis and design to insure the most effective heat flow and fuel economy for the steam generator. The relationship between rising heat flow from the burner(s), flue gas passage, and the spacing between the multiple tubing loops is critical to uniform and effective heat transfer within the vapor generator component.
It is imperative that the working fluid/water-to-steam residence time within the steam generator be no more than seven seconds for automotive applications where quick startups are required. Other end applications may tolerate longer start-up times but this is not true for any automotive application if a nearly competitive stance is to be maintained with the current, conventional I.C. engine. A range of fluid residence time of between five and seven seconds must be the target for an acceptable vapor generator/burner assembly for these newer Rankine cycle engine systems.
Although the burners of the Rankine cycle engine system may be fired with nearly any suitable solid, liquid or gaseous fuel, it is expected that only a few liquid fuels will be considered practical for most applications. The first probably choice as a burner fuel will be kerosene because of its relatively low cost and general availability. The second suitable choice would be the alcohols--methanol and ethanol, which although are not now generally available should emerge as practical and economical fuels in the future, as world petroleum reserves are depleted. These are attractive natural, renewable fuels.
The use of a secondary fuel along with any given primary fuel has not been given much consideration for engine systems but offers some useful possibilities when the secondary fuel can be reclaimed from the engine exhaust, or by some form of fuel and exhaust reforming and/or regeneration. As an externally fired closed cycle--the Rankine closed cycle turbine system is particularly well adapted to the use of a secondary fuel since no special dual-fuel mixing provisions are needed in the external combustion process.
The secondary gaseous fuel of a closed Rankine cycle system may be readily and directly mixed and burned with any primary liquid fuel to increase the open flame intensity, or to allow the primary fuel flow to be cut back slightly as a fuel conservation measure.
The final major Rankine cycle component which requires by far the most improvement and development effort is the vapor or steam condenser which causes the expanded/expended steam or vapor flow to be changed or condensed back to its original liquid state. Some effort has been made to improve this c/c component but most of the work has failed to overcome its most prominent shortcoming which is excessive vapor flow impedance which both lowers the efficiency of the expander/engine due to pressure relief blocking and power takeoff for the cooling fan. Any attempt to use standard automotive radiators with large cooling fan(s) in a "brute force" arrangement to achieve rapid condensation is both wasteful and self-defeating to the end result. The use of the conventional automotive radiator as a Rankine cycle component is not an acceptable and practical way to cost/effective Rankine cycle operation and negates the improvements which can be made in the other major c/c components.
The ideal Rankine cycle engine condenser must rapidly condense the spent hot vapor or steam, preferrably by an expansion technique which will insure that vapor flow impedance is fully avoided or reduced to a minimum impedance effect on the turbulent vapor or steam flow within the closed loop.
The expansion condenser concept is not new having been advocated for use as a major Rankine cycle component in 1971, with a U.S. patent application filed in January of 1972. The "Helical Expansion Condenser" pending application--Ser. No. 219,985, has been granted as of March of 1977.
Essentially the expansion condensing device consists of providing a uniformly increasing duct or chamber volume into which the expended vapor or steam from the engine/expander is conducted. The uniform cooling and condensation of the vapor/steam is accomplished in two ways--firstly by conventional heat conduction and radiation thru the condenser tubing walls and--secondly, by uniform expansion whereby the expanding spent vapor/steam is proportionately cooled.
The expansion cooling effect of the expansion condenser aids the primary conduction and radiation cooling effect by uniformly reducing the vapor flow velocity which increases the vapor/steam residence time within the expansion condenser and the corresponding effectiveness of the conduction and radiation ducting means.
An important element towards the success of the expansion condensation concept and hardware is the addition of both internal and external multiple, thin fins within the larger cross-sectional zones of the duct(s) in order to sink heat at a rapid rate proportional to the uniformly increasing cross-sectional area and volume. As the expanding vapor/steam flow loses velocity quickly a threshold point is reached where condensation will begin and continues until the vapor/steam flow fully condenses at close to zero velocity within the end portions of condenser where it can be pumped back as a liquid into the vapor generator to repeat the continuous phases of the closed Rankine cycle.
If each of the major components of the closed Rankine cycle system are built according to the general outline of this background description a practical and cost/effective c/c engine system will result which would be applicable to a wide range of both mobile and stationary power needs.